In connection to the compacting of a powder to a green body during production of cutting inserts, the powder is introduced into a cavity defined by a die. Normally, the die comprises an upper opening through which the powder is introduced into said cavity and through which, during a subsequent pressing step, an upper punch is introduced into the die. Typically, there is also provided a lower punch which is able of sliding through a tunnel, often referred to as a bore or pressing bore, in the die and which will form at least part of a bottom of said die cavity and by means of which the green body formed upon compaction of the powder is ejected from the surrounding die. From the upper opening of the die there is provided a punch tunnel, also often referred to as a bore or pressing bore, in which the upper punch is able to move downwards for the purpose of contacting the powder and subjecting it to a compacting pressure. In other words, the punch tunnels contribute to the definition of the die cavity provided for receipt of the powder, and the punches are provided for the purpose of compacting a powder received in said cavity.
After compaction of the powder into a green body, the upper punch is retracted out of the die, and the green body is ejected by a motion of the lower punch relative to the die (either of these components could be the one which is moving). Accordingly, the green body is ejected through the tunnel in which the upper punch was moving downwards into the die during the pressing step.
Typically, the material of the green body is of such nature that it will expand when the compacting pressure from the punches is relieved and when it is ejected and released from the surrounding die. In order to enable the green body to expand radially when it is ejected from the cavity in which it has been compacted, the tunnel is widened slightly above the level to which the upper punch is forwarded during compaction. It could be said that the cavity is provided with a release portion having a certain inclination angle of the inner wall of the die relative a centre axis of the cavity. The inclination angle (possibly also referred to as the release angle) as well as the length (in the vertical direction) of the release portion is adapted to the expected (radial) expansion of the green body upon ejection thereof.
Typically, for positive inserts the cavity has a cross section that narrows as seen from the release portion to the remaining (lower) part of the cavity. Upon compaction of the powder, a lower edge of the upper punch, which is defined by the intersection of a lateral surface and a bottom surface thereof, is not allowed to come into contact with the inner peripheral surface of the die, since such contact might result in damages on both die and punch. Therefore, the punch is only forwarded to a level at which there will be a small gap between said punch edge and the inner peripheral surface of the die. During compaction of the powder, the latter will to some extent leak out and to some extent be extruded out through said gap and into the release portion. The leakage will result in gathering of loose powder in the release portion and the extrusion and/or leakage will result in a residual edge being formed on the green body along the upper edge thereof, which needs to be treated, i.e. at least partly removed, before the subsequent sintering of the green body. Such treatment is time-consuming and contributes to unwanted production costs. The loose powder gathered as a result of said leakage, on the other hand, results in unwanted loss of material and will have a negative influence on the surface finish of the compacted body as the latter is ejected out of the die, especially if the radius of the edge of compacted body is very small and thereby sensitive to interaction with loose powder. Further negative effects of powder leakage and extrusion may also be an unwanted effect on the shape of the upper edge of the green body or a lower density, i.e. a generation of porosity, in the region of the upper edge of the green body.
The present applicant has considered a solution, to the above-mentioned problem, according to which the die is subdivided into an upper part and a lower part, and wherein the die cavity that will define the geometry of the green body in the final compaction position of the punches is housed in the lower die part. During compaction, the upper punch is advanced through the upper die part and into the lower die part. The parting plane between the upper and lower die parts is just slightly above the level to which the edge of the upper punch is advanced in the final compaction position thereof. During filling of the die with powder, the upper die and the upper punch is held in a retracted position above an upper opening in the lower die part. The parting plane is generally flat and a powder-filling device is permitted to slide on an upper surface of the lower die part to and from a position on top of said upper opening. Typically, the powder is filled to the same level in the die as the upper surface of the lower die. After filling of powder, the upper die is joined with the lower die, and compaction is performed by means of the lower punch and the upper punch. When the compacted green body is subsequently to be displaced out of the die, the upper die part is removed from the lower die part before said displacement of the green body. Thanks to this principle solution and design of the die, the release portion into which powder may leak and form a residual edge during compaction can be kept very small.
Preferably, the length of the release portion, i.e. the distance in the direction of the pressing axis from the punch edge (in its final compaction position) to an edge that defines the upper opening in the upper surface of the lower die part should be generally the same all around the circumference of the punch. Therefore, in an application in which the punch edge of the upper punch is partly non-perpendicular to the pressing axis of the upper punch, said edge that defines the upper opening in the upper surface of the lower die part should be correspondingly partly non-perpendicular. If, for example, the punch edge has a wave-like extension, i.e. a varying position in the direction of the pressing axis around the circumference of the punch, the edge that defines the upper opening in the upper surface of the lower die part should have a corresponding wave-like extension. As a result thereof, the upper surface of the lower die will not be planar, and will not allow a powder-filling device having a generally plane bottom surface to be slid to a position on top of the upper opening in the lower die part and fill the lower die part with powder to the level of the edge that defines said upper opening. There is also a risk of having powder escaping to the upper surface of the lower die part in connection to such a filling.